The proposal will define the mechanism by which restraint stress inhibits the testicular response to gonadotropin in male rats and determine the involvement of peptides produced in the testes (ACTH, beta-endorphin, alpha-MSH and arginine vasopressin) in mediating this process. Experiment 1 will examine the effect of restraint stress on testicular steroidogenesis in vitro. The effect of restraint on LH bioactivity and LH receptors on interstitial cells will be determined. Steroid production from cultured interstitial cells will be assessed to identify specific steps in the steroidogenic pathway that are altered by stress. The effect of restraint stress on the activity of key steroidogenic enzymes will be determined. HDL and LDL receptor binding will be compared between isolated interstitial cells from stressed and unstressed rats. Experiment 2 will examine the effect of stress-sensitive- pituitary peptides and glucocorticoids on the same parameters in cultured interstitial cells from unstressed rats. The influence of stress on the volume of testicular interstitial fluid (TIF) and the level of opiomelanocortin peptides and arginine vasopressin in TIF will be determined in Experiment 3. Experiment 4 will examine the effect of incubating TIF from stressed and unstressed rats on steroid production by cultured control intersititial cells. Specific antagonists of peptides found in TIF will be incubatied with TIF from stressed and unstressed rats and the effect on steroid production by control interstitial cells determined in Experiment 5. In Experiment 6, the effect of selective destruction of Leydig cells on the ability of TIF from stressed rats to alter steroid production by control interstitial cells in vitro will be assessed to determine the source of the active regulatory factors in TIF. These experiments will define the mechanism of stress-induced testicular hyposensitivity to gonadotropin and the role of testicular peptides in this regulatory process.